This invention relates to a singlemode optical waveguide fiber with improved bend performance.
Bend loss is a phenomenon in which a portion of the light travelling through an optical waveguide fiber is lost due to physical bending of the fiber. Part of the light travelling through the core region of a fiber is stripped off at a bend in the fiber, causing that light to be lost. A discussion of bend loss can be found in Miller et al., Optical Fiber Communications, pp. 62-65, pp. 92-98, pp. 158-161, Academic Press, New York, 1979.
Bend loss is particularly a problem for applications using ribbon cables. A ribbon sub-unit is a linear array of individual optical fibers contained in a protective sheathing. A number of these ribbon sub-units may be stacked and placed into a larger cable sheath along with strength members to form a ribbon cable. Ribbon cables are extremely space efficient and can contain a large number of fibers. Typically, 4 to 16 fibers are in a ribbon sub-unit, and 12 to several hundred ribbon sub-units are combined to form a ribbon cable.
Because of the arrangement of a large ribbon cable, the outside fibers (edge fibers) in each individual ribbon subunit may be exposed to large bends or twists during fabrication of the individual ribbon sub-unit or of the larger combination of individual ribbon sub-units in cables, or during installation of the finished cable. The bends in the fibers within a ribbon cable may result in large bend losses if the optical fibers are bend sensitive. If the fibers exhibit large bend losses, the system using such fibers will exhibit higher losses. This is especially problematic in situations where the optical power budget (the amount of loss allowable) is tight. Also, because bending of an individual fiber is unpredictable and inconsistent, the amount of bend loss from fiber to fiber may differ substantially.
The bend loss of a singlemode optical fiber is determined by its mode field diameter, MFD, and cutoff wavelength, .lambda..sub.c. As illustrated in FIG. 1, the ratio of MFD/.lambda..sub.c can be used as an indicator of the bend loss of the fiber. Decreasing MFD will result in more concentration of the optical power distribution in the center of the fiber. This concentration of optical power results in less optical power which can be lost at a fiber bend, thereby reducing the bend loss.
Increasing .lambda..sub.c will also reduce the amount of power which will be lost at a bend for a given power distribution. Because an optical waveguide fiber must be single mode at about 1310 nm, .lambda..sub.c cannot be raised much above 1320 nm. Therefore, significant improvement in bend loss can result only from lowering MFD.
Raising the refractive index delta, .DELTA., in the core region of the fiber is well known in the art as a method for decreasing the MFD. However, raising A in the core region can result in an unacceptable increase in .lambda..sub.c or an unacceptable increase in the zero dispersion wavelength, .lambda..sub.0, at 1310 nm. one known method for maintaining .lambda..sub.c while decreasing MFD is simultaneously to increase core A and reduce the radius of the core region. This will result in an unacceptable increase in .lambda..sub.0. Also, raising the core .DELTA. in a step index profile is more difficult to manufacture and may cause higher attenuation in the fiber as a result of increased Rayleigh scattering caused by the higher dopant concentrations required to raise the refractive index.
There is a shallow manufacturing window in which MFD, .lambda..sub.c and .lambda..sub.0, are within acceptable ranges. For practical applications, MFD should be below about 9.8 .mu.m, .lambda..sub.c should be between about 1200 and 1320 nm, and .lambda..sub.0 should be between about 1301 and 1321 nm. Varying core .DELTA. in a step index profile singlemode optical fiber readily moves one outside this manufacturing window such that .lambda..sub.c and .lambda..sub.0 are no longer within acceptable ranges.
Bhagavatula U.S. Pat. No. 4,715,679 discloses numerous refractive index profiles which include a core with inner and outer regions separated by at least one region of depressed refractive index. Bhagavatula further discloses that by altering the radial location, width, depth and shape of this region of depressed refractive index, fibers can be designed with specific waveguide dispersion characteristics. Bhagavatula also discloses that if the core radius is made too small in order to balance out material dispersion, unacceptably high microbending losses will occur. Bhagavatula only discloses inner core regions with diameters greater than 40%. There is no disclosure in Bhagavatula regarding a diffusion tail between the core and cladding regions of the fiber.
Nakahara et al. Japanese Patent Application No. 51-134138 discloses a singlemode optical fiber with a refractive index profile in the core region that includes a refractive index maximum at both the center and circumference of the core. Nakahara et al. discloses that this core refractive index profile allows the core diameter to be made larger, allowing for easier splicing. Making the core diameter larger will increase the MFD of the fiber, resulting in higher bend losses.
Kawana et al. Japanese Patent Application No. 53-97849 discloses a singlemode optical fiber with a refractive index in the center of the core region which is higher than the refractive index of the outer portion of the core region. Kawana discloses that the radius of the center of the core region is less than 50% of the core radius to limit the increase to .lambda..sub.c. The benefits disclosed include lower losses as a result of less leakage of electromagnetic field into the clad portion of the fiber and lower bend losses. There is no disclosure in Kawana et al. regarding the effects of the profile design on MFD or .lambda..sub.0. There is also no disclosure in Kawana et al. regarding a reduced germania diffusion tail at the interface between the core and cladding regions or any ring of increased delta at the outermost edge of the core region.
Reed U.S. Pat. No. 4,852,968 discloses another method for achieving reduced bending losses in singlemode optical waveguide fibers. Reed discloses a singlemode optical fiber with a core region, a first cladding region surrounding the core region, a trench region surrounding the first cladding region, and a second cladding region surrounding the trench region. The refractive index of the trench region is lower than the refractive indices of the first and second cladding regions. Reed discloses that the presence of the trench region results in lower bending losses as compared to fibers without the trench region. However, producing fibers with several cladding regions of different refractive indices increases the cost and complexity of producing such fibers as compared to fibers with a single core region and a uniform cladding profile.
J. C. Lapp et al., "Segmented-Core Single-Mode Fiber Optimized for Bending Performance", J. of Lightwave Technology, vol. 6, no. 10, pp. 1462-65, October 1988, discloses a fiber with a segmented-core profile of the type disclosed in Bhagavatula U.S. Pat. No. 4,715,679. The profiles in Lapp et al. are optimized to improve bend performance. The segmented-core profile of Lapp et al. consists of: i) an inner core region of high delta, ii) an intermediate core region of depressed delta, and iii) an outer core region of high delta. Lapp et al. discloses deltas for the inner and outer core regions ranging from 0.4 to 0.5% and delta for the intermediate region of 0.1 to 0.2%. Lapp et al. further discloses diameters of the inner core region of about 70 to 90% of the diameter of the entire core region of the fiber and thicknesses of the intermediate core region of depressed delta in the range of about 0.2 to 1.0 .mu.m. As will be discussed further in the present application, .lambda..sub.c is increased dramatically when the diameter of the inner core region is greater than about 60% of the diameter of the entire core region of the fiber. Also, it would be difficult to repeatably manufacture fiber with the profile disclosed in Lapp et al. because of the tight dimensional control required for the narrow intermediate core region of depressed cladding. Poor control of the delta in the intermediate core region results, in part, from diffusion of dopant materials from both the inner and outer core regions caused by the higher concentrations of dopants in those two regions relative to the intermediate core region. There is no disclosure or suggestion in Lapp et al. regarding a diffusion tail between the core and cladding regions of the fiber.